Mastrobuoni is not in the Mariners' starting lineup against the Guardians on Friday.
Mastrobuoni has been out of the lineup in three of the Mariners' last four games, and his playing time could diminish further with Luke Raley (oblique) close to returning from the 10-day injured list. Randy Arozarena, Julio Rodriguez and Dominic Canzone will start in the outfield for Seattle.